1. Field of the Invention
This invention relates in general to a double reflector antenna and in particular to a Cassegrain or Gregory type antenna for the frequency range above 6 GHz.
2. Description of the Prior Art
Double reflector antenna which use a main reflector dish and a sub-reflector mounted at the focal point of the main reflector generally utilize a primary for field exciting means in the form of a small horn radiator which has its free end projecting through the central opening of the main reflector and is mounted between the main reflector and the sub-reflector. Such antenna are used for radio relay or satellite relay systems and a double reflector antenna of this type is described, for example, in the publication Siemens Zeitschrift, "Nachrichtenuebertragungstechnik" Supplement, 48th edition, 1974, pages 226 to 229.
Such arrangement of the primary for field radiator mounted between the main reflector and the sub-reflector requires necessarily that long supply lines be utilized between the radiator and the transmitting-receiving equipment which are generally contained in a housing behind the main reflector. The attenuation of wave-guide supply lines which are long gives rise to undesirable attenuation which is noticeable during transmitting operation. During receiving operations, the relatively high attenuation of the wave-guide supply line makes it necessary that the preliminary amplifier be mounted in the immediate vicinity of the feed point of the primary for field radiator and, for example, when a horn radiator is used inside the supporting structure of the reflector. Also such arrangement may produce an adequate reduction in the attenuation loss of the wave-guide supply line through the transmitting-receiving equipment for a receiving operation the access for servicing and assembly to the pre-amplifier is very poor.
The radiating opening of the primary far field radiator aperture must be protected from weather influences by means of a thin dielectric foil, but in the prior art devices it is directly exposed to rain or snow. At higher frequencies, layers or drops of water, snow and ice on the foil produce serious disturbances and impairment of the operating properties due to the reflection and absorption of the signal by the water, snow and ice. This becomes particularly disturbing in antenna which operate according to the principle of so-called frequency refuse systems. It has been learned from experience that the higher requirements which must be maintained in order to obtain the accuracy of the cross-polarization of the antenna so as to ensure satisfactory operation cannot be maintained under such conditions. For this reason, in the prior art it has been common practice to generally provide fans which keep the radiator opening free of water or snow deposits.